This invention is generally related to maximizing window efficiency and enabling control of the transmission of solar radiation into the interior of a building. This invention more specifically relates to advanced smart windows for high energy efficiency and recycling capability which include a diffraction grating for separation of different spectral regions for selective rejection and/or transmission of infrared, visible and ultraviolet radiation into the interior of a building. This invention also relates to a smart window having a selectively patterned micro cell array that aids in reflecting incident light having a specific range of wavelengths, dispersed by a diffraction grating, from the solar spectrum.
White light from the sun has a broad range of wavelengths. Among them, visible rays with wavelengths ranging from 0.4 μm to 0.7 μm are always useful to humans, while infrared rays with wavelengths ranging from 0.7 μm to 3.0 μm are only useful for some seasons. Approximately 40% of total solar flux lies in the infrared spectral region. In the summer season, it is frequently desired to reject the entry of solar heat into a building to reduce the building cooling costs, whereas in winter season it is desirable to permit the solar IR radiation to enter the building to reduce the heating costs. The function of conventional energy saving window technologies is currently confined to simply blocking the solar radiation without considering the wavelength of the incident light, so this leads to a need for extra interior lighting even in daylight hours.
From an optics perspective, conventional windows are classified as passive devices that function depending upon inherent characteristics of the glass and polymeric materials of which they are constructed. With conventional windows, mechanical methods are generally used to physically block direct solar radiation. Typical examples include awnings, louvers, blinds, etc. which usually have a fixed construction but can be made adjustable or retractable in response to changes in solar radiation direction. Louvers and blinds are typically composed of slats varying in size, width, and shape depending on the application. They are mainly intended for shading, but can also be used to redirect daylight, reduce glare, and control solar heat gain. Fixed mechanical systems are usually available at a low cost, but controllable mechanical systems can be more expensive. A major drawback is that mechanical methods typically block daylight when only heat-blocking is desired.
Passive optical methods have also been utilized for redirecting daylight, reducing glare, and controlling solar heat gain. Spectrally selective coatings applied to window glass typically reject a specific fraction of the solar spectrum, generally ultraviolet and infrared radiation, while admitting visible radiation. In this way, such a coating can allow visible light to be transmitted through a window, but blocks heat-generating ultraviolet and/or infrared radiation. Spectrally selective coatings are typically composed of thin metal films or a dielectric-metal multilayer which is coated or applied to the glass window. Such coatings can reduce solar heat gain, and thus directly benefit buildings situated in hot climates where the cooling load is a major energy cost. They can also be applied inside of a building to reduce heat loss through the windows in cold climates but will also block incoming heat from direct daylight, which is an important natural heating source. This technology, however, is not ideal for use in mixed climates because the coating property is fixed once it has been applied to the window.
A smart window, however, is an active device which can control optical transmittance by application of a set of electric signals. Various active window systems exist, all of which have major limitations. For example, one technical approach for smart windows is mainly limited to changing only one optical property, transmittance, for all ranges of wavelengths.
A photochromic window is one example of such a smart window. Photochromic windows experience a photochemical reaction under exposure to solar radiation in a specific spectral range, usually UV. This reaction changes the optical absorption band of the window, which is originally transparent to visible radiation, resulting in a change of color. The reaction can be reversed by eliminating the light source that has activated the transformation. The optical transparency is automatically varied with the intensity of the incident light. Such a window has the weakness that it can work only depending on the intensity of the external light, thereby not meeting seasonal requirements; for example, in summer months it blocks infrared radiation as well as useful visible light. Photochromic technology is widely known for its use in sunglass lenses.
Similarly, thermotropic or thermochromic windows change their optical properties in response to a temperature change. In general, they are transparent at lower temperatures and become translucent or opaque at higher temperatures. The basic mechanism is based on the movement of the component molecules to cause a phase change in the material, which scatter light accordingly. The major commercial applications of these materials are skylights and upper windows where visual comfort can be ignored.
Photochromic, thermotropic, and thermochromic windows are self-regulating, which make them less useful as energy saving devices since they can not be manually controlled to respond to the changing environment. Their optical properties can change when exposed to UV radiation and/or altered temperature. Photochromic materials will block heat on a sunny cold winter day, and thermotropic materials will block visible light on a warm summer day.
Active materials, such as those used in liquid crystal displays have an advantage over the use of photochromic and thermochromic materials in smart windows since they are electrically programmable and switchable. Commercially available liquid crystal displays are typically composed of two polarized glass substrates with a liquid crystal region between them and have transparent oxide electrodes. The first glass substrate is polarized in one direction and the second one is polarized in a perpendicular direction. In the off state, the liquid crystal molecules rotate the polarization of the light by 90 degrees, allowing incident light to pass through the two glass substrates without optical loss. When an electric field is applied, the liquid crystals will align and no longer rotate the polarization of the incident light; thus, the display will be opaque.
A polymer dispersed liquid crystal (PDLC) material is another useful liquid crystal system; however, the liquid crystal structure used in PDLC smart windows is somewhat different from that of a liquid crystal display. In a PDLC smart window, an emulsion of a polymer and liquid crystal is formed into a film. The refractive index of the polymer matrix is matched to the dispersed liquid crystal. The film is then sandwiched between two transparent sheets which are coated with a transparent conducting material, such as Indium Tin Oxide (ITO). In the off state, randomly oriented liquid crystals scatter light, making the film translucent. When an electric field is applied, the dispersed liquid crystals align parallel to the field and change the film's transparency. These windows are suitable for privacy windows, as they do not sacrifice light; however, they cannot efficiently block heat. The lack of a memory effect requires continuous power to hold the window in a transparent state; therefore, the power consumption is ultimately higher than that of other materials which only require power during switching.
Suspended particle system (SPS) windows utilize a similar concept as PDLC windows except that they use light absorbing microparticles instead of scattering liquid crystals to make the window opaque. SPS windows have an active polymer layer where the light absorbing microparticles are suspended. This layer is sandwiched between two sheets coated with transparent conductors, for example ITO, with a dielectric layer on top. In the off state, the suspended particles are randomly distributed and absorb light, making the film opaque. By applying an electric field to the active layer through the transparent conductors, the particles align to the field and change the transparency of the film. An application similar to SPS is electrophoretic electronic paper, also know as E-ink, which utilizes the migration of color coated suspended particles under the influence of an applied electric field. Since heat and light are both absorbed, the major application of SPS windows is shading, and illumination may be required inside a building, increasing energy use.
Electrochromic windows are among the most technologically advanced window systems. Instead of using suspended particles or liquid crystals, electrochromic materials are typically composed of a stack of an electrochromic layer, a conducting electrolyte layer, and an ion storage layer, all of which are placed between transparent conductors, such as ITO. They are transparent in the off state, and optical properties of the electrochromic layer can be changed by the injection of coloration ions from the storage layer. An applied electric field drives ions from the storage layer through the conducting layer to the electrochromic layer, altering the electronic structure of the electrochromic layer. This reaction switches the window from the transparent state to the opaque state. A reverse electric field will draw the coloration ions back into the storage layer switching the electrochromic layer back to its original clear state. Various coloration ions such as Li+, H+, Na+, and Ag+ can be utilized. Inorganic oxides such as WO3, NiO, V2O5, and MoO3 can be used for the electrochromic layer, among which WO3 has been most widely studied. Electrochromic windows only consume power during switching, require a low driving voltage (1-5V) and have long term memory (12-48 h), making this technology energy efficient. However, fabrication of large area windows is very expensive (˜$50-100/ft2) and illumination is still required since the window absorbs or reflects the visible light.
Gasochromic materials share the principle of electrochromic materials except that the coloration ions are supplied by means of gas. Hydrogen (H2) gas is typically injected between two panes, where one of the panes has a coating of a thin catalytic layer on top of a chromogenic layer, for example WO3. Decoloration can be achieved by feeding another purging gas. The major drawback with this technique is the need of integration of gas lines into the window which is a large construction limitation.